libcommon|Hexen: Moved the internal savegame to /home/cache/internal.save

Hexen's internal savegame is no longer erroneously listed in the UI
however it is still written to disk as before.

Note that any legacy internal savegame in the old hexndata folder
will still be converted automatically, placed into the respective
/home/savegames subfolder and will therefore appear in the UI still.